Testing of Concrete in Structures: Fourth Edition

94 Partially destructive strength testswhich is cast into the concrete, and those which offer the greater flexibilityof an insert fixed into a hole drilled into the hardened concrete. Castinmethods must be preplanned and will thus be of value only in testingfor specification compliance, whereas drilled-hole methods will be moreappropriate for field surveys of mature concrete. In both cases, the valueof the test depends upon the ability to relate pull-out forces to concretestrengths and a particularly valuable feature is that this relationship isrelatively unaffected by mix characteristics and curing history. Althoughthe results will relate to the surface zone only, the approach offers theadvantage of providing a more direct measure of strength and at a greaterdepth than surface hardness testing by rebound methods, but still requiresonly one exposed surface. Procedures have recently been reviewed in detailby Carino (103).4.2.1 Cast-in methodsReports were first published in the USA and USSR in the late 1930s describingtests in which a cast-in bolt is pulled from the concrete. These methodsdo not appear to have become popular, and it was not until 30 years laterthat practically feasible tests were developed. Two basic methods, bothof which require a threaded insert which is fixed to the shuttering priorto concreting, have emerged. A bolt is then screwed into the insert andpulled hydraulically against a circular reaction ring. The principal differencebetween the two systems, developed in Denmark and Canada respectively,lies in the shape of insert and loading technique. In both cases acone of concrete is ‘pulled out’ with the bolt, and the force required toachieve this is translated to compressive strength by the use of an empiricalcalibration.4.2.1.1 The Lok-testThis approach, developed at the Danish Technical University in the late1960s, has gained popularity in Scandinavia and is accepted by a numberof public agencies in Denmark as equivalent to cylinders for acceptancetesting (1). It has subsequently gained wide international acceptance as amethod for demonstrating adequate strength for early formwork stripping.Time savings of 30% and labour savings of 45% have been claimed withstripping taking place in as little as 19 hours. The potential for use infast-track construction has been recognized in the UK by a Best PracticeGuide (104).The insert (Figure 4.9) consists of a steel sleeve which is attached to a25 mm diameter, 8 mm thick anchor plate located at a depth of 25 mmbelow the concrete surface (105). The sleeve is normally screwed to the